Currently, certain transportation systems are based upon the use of passenger and freight carrier units which are propelled by drive wheels that frictionally engage a drive-shoe assembly mounted to the carrier units. In order to insure relatively positive control and frictional driving of the carrier units, such transportation systems typically support a substantial portion of the weight of each unit by engagement of the drive-shoe assembly with the drive wheels as the units pass along the guide track or rail. People mover systems having rail-supported cabins or cars, and aerial tramways with detachable chairs or gondolas, are both examples of transportation systems in which the carrier unit weight often is supported on the system's drive wheels as the units are advanced.
One example of a people mover system of this type is disclosed in my U.S. Pat. No. 4,671,186 entitled, "Positive Drive Assembly for Automatic, Rail-Based Transportation System." In such transportation systems the drive-shoe assemblies must be of a sufficient length to serve a two-fold purpose.
First, the drive-shoe assembly must always be in contact with at least one drive wheel so that the carrier unit, containing the people or cargo, may be accelerated up to speed from a stopped condition at any location along the support rail or track. Thus, the drive wheels must be able to propel the carrier unit from a stationary position, not only at predetermined stations so that people or cargo may enter or exit the carrier unit, but also at any location along the rail so that the system can be started again after a power failure or after an emergency stop.
Secondly, the drive-shoe assembly must be of a sufficient length to support a substantial portion of the weight of the carrier unit, especially when it is full. This second requirement has resulted in the use of drive-shoe assemblies which were so long that they were in contact with several drive wheels at the same time. This presented a problem to which the present invention is directed.
In the transportation system of my prior patent, U.S. Pat. No. 4,671,186, carrier units were accelerated up to speed (and decelerated) by operating adjacent drive wheels at different speeds and allowing at least one wheel to slip with respect to the drive-shoe as the units were accelerated. The first wheel encountered by the drive-shoe would essentially control the unit speed. When the drive-shoe engaged the next wheel, operating at a higher speed, the next wheel would initially slip with respect to the shoe and then accelerate the unit up to the speed of the high speed wheel, which would result in slipping of the slower moving first wheel with respect to the drive shoe.
It was discovered, however, that as the carrier unit weight and payload increased, the drive-shoe must engage more adjacent drive wheels in order to support the carrier load. In the acceleration/deceleration sections (not the sections of constant velocity) of the transportation system, the differing operating speeds of adjacent drive wheels was producing slipping over such a substantial length of the longer drive shoe that the drive wheels (rubber tires) were wearing out prematurely. Additionally, as the weight of the carrier unit increases and the drive shoe contacts more drive wheels, the change in speed (acceleration) of the carrier unit from one drive wheel to the next is somewhat less smooth. The high weight and corresponding high friction force on each drive wheel tends to cause fighting of the drive wheels to control speed before one of the wheels begins slipping.
Accordingly, it is an object of the present invention to provide a drive-shoe assembly and a method suitable for use in a transportation system which are capable of supporting a substantial carrier unit load while maintaining a smooth acceleration/deceleration of the carrier unit and while minimizing drive wheel wear.
Another object of the invention is to provide a drive-shoe assembly and method which reduce the amount of sliding frictional contact between the drive shoe assembly and adjacent drive wheels operating at different speeds.
It is also an object of the invention to provide a drive-shoe assembly and a transportation system which will support the weight of the movable carrier units while reducing drive wheel wear.
It is a further object of the invention to provide an apparatus and method, consistent with the foregoing objects, which is durable, easy to install and maintain and economical to manufacture.